docs(demo): compare.html says what untrusted checks

[alekswebnet]

Closes #28.

Two passages on /demo/compare.html described the player as it was before untrusted (0.8):

• "When to choose something else": the bullet saying the H5P plugins check uploads "and this player does not" is replaced by the difference that remains: the plugins install new library code from an administrator's upload, while the pack is pinned per release, so a package needing a newer library waits for the next pack.
• "Data and trust": "This player checks none of that" now says what untrusted does (no package code, the plugins' file-type list for content/, the content checked by a port of the plugins' validator, compared on 98 cases), links docs/untrusted.md, and keeps Embed as the extra layer.

Release

None: demo site only. Vercel deploys it on merge.

🤖 Generated with Claude Code

[alekswebnet]

Closed: docs-only changes go straight to main, no PR.